Multiple-access communications system

ABSTRACT

A multiple access communications system for use between a base and a plurality of substations and having a first channel for transmission of data from the base to the substations and a second channel for transmission of data from the substation to the base. The system has, in use, provision at the base and/or substations for assigning each of the substations to one of a plurality of groups, and for allocating, from the base, each of a plurality of slots in the second channel to a particular group. There is provision at the base for reporting to the substation via the first channel the number of substations in each group, and there are analyzing means at each substation for analyzing the number of substations in each of the groups and for making a judgement based on those numbers as to which group to assign each respective substation to in order for that substation to communicate next with the base in the slot or slots allocated to that group.

TECHNICAL FIELD

This invention relates to multiple access communication systems and amethod of operating the same, especially such systems which allowcommunication of data between one or more base stations and a pluralityof substations or mobiles.

Multiple access communication systems and in particular those employingmobile radio channel assignment are well known. Problems associated withsuch systems include difficulties in allowing the substations randomlyto access the base station especially at busy periods. Simultaneousattempts at access by two or more substations can lead to a collisionand mutilation of the data in question and subsequent attempts at accesscan cause an increasing number of collisions, further mutilation andultimately instability of the whole system.

A polling system under which a substation communicates with a basestation only on request by the base station can be secure and allow highcapacity of use but suffers from the disadvantage of lengthy delays forany substations waiting to transmit.

Various attempts have been made to try to overcome the problemsassociated with wholly random access systems and with polling systemsand these have centered on controlling the timing and/or manner ofaccess of the base station by the substation through the use ofprearranged protocols.

One of the first such protocols was termed "Pure Aloha" under which asubstation transmits a request to the base station to transmit data andwaits for an acknowledgment of the request. If none is forthcoming, thesubstation waits a random time before resubmitting the request.

A modified protocol was termed "Slotted Aloha" under which users of thesystem were allowed to transmit requests within a discrete timeslot. Arelated protocol was termed "Framed Aloha" under which the base stationtransmitted a message on a signalling channel to indicate whichtimeslots (arranged within "frames" containing a predetermined number oftime slots) would be available to substations for making requests.However, all such systems generally were not particularly efficientunder normal traffic conditions and became unstable under heavy trafficconditions.

A still further protocol termed "Dynamic Frame Length Aloha" attemptedto take account of varying traffic conditions by varying the number oftimeslots per frame on the basis of an evaluation of the level of usemade of the previous frame. However, there were again problems with thisprotocol because of the limitations dictated by the system on the degreeof variation of the number of slots in a frame and on the complexity ofthe systems employing the protocol.

DISCLOSURE OF THE INVENTION

The present invention is based on a protocol which can deal effectivelywith multi-access communication without resort to the complexity of avariable number of slots per frame and generally provides an efficientcommunication system.

In accordance with the invention, there is provided a multiple accesscommunications system for use between a base and a plurality ofsubstations and having a first channel for transmission of data from thebase to the substations and a second channel for transmission of datafrom the substation to the base, wherein the system has, in use:

i) means for arranging for the substations and prospective substationsto be split into a plurality of groups,

ii) means for assigning each of a plurality of slots in the secondchannel to a particular group,

iii) means for the base to inform the substations via the first channelof the number of substations in the groups, and

iv) means for each substation to analyze the numbers of substations ineach of the groups and to make a judgment based on these numbers as towhich group to associate itself with in order to next communicate withthe base in the slot or slots assigned to that group.

On joining the system, substations may be assigned by the base to thegroups on a rolling basis, but substations leaving the system during anyparticular period of operation are not automatically replaced by anothersubstation joining the system. In this way, there tends to be a random,varying, number of substations in each group at a given time.

Alternatively, however, on joining the system, a substation may assignitself to a particular group by reference to the numbers of substationsin each group.

In general operation of the system, each substation is entitled randomlyto attempt to communicate with the base in the slot assigned to itsgroup.

As with other random access systems, any communication from a substationto the base station which is not acknowledged because, for example, ofmutilation caused by collision between two substations of the same groupattempting simultaneously to use their common assigned slot, wouldnormally be re-transmitted.

In the event of failure to transmit, each substation knows from the basethe number of substations in each group and hence, on the basis thatcollisions are least likely in the lowest populated groups, knows theslots it can use with the greatest chance of successful transmission.

The system is set up, therefore, to allow a substation in a given groupto use slots assigned to other groups in the event of collision or otherunsuccessful transmission to base.

Preferably, however, because of the likelihood of other substations(especially the one causing the collision with the substation inquestion) trying simultaneously to use a slot of the lowest populatedgroup, the system possesses a random number generator or other suitablemeans to determine either the number of timeslots which must pass beforethe next attempt to transmit is made, and/or which slot or slots from a"pool" of slots of low population numbers are used to make the nextattempt to transmit.

There may be occasions when traffic flow in the system is beingdisrupted owing to, for example, too many substations attempting to useslots assigned to a particular group or to there being a general backlogof data waiting to be transmitted.

In one embodiment, therefore, means are built into the system to allowthe base artificially to amend the number of substations in a group soas to deter or encourage use of slots assigned to that group, especiallyby substations outside the group in question. Thus, if in a group havinga maximum population of sixteen substations the actual number ofsubstations is eight, the base may artificially indicate a populationof, say, fifteen or sixteen to deter use of slots assigned to thatgroup.

In another embodiment, the system is designed so that, under normaltraffic flow conditions, a proportion of the theoretical number ofavailable groups (hereafter called "special" groups) remains unused,i.e. no substations are allocated to them. This can be done by assigningan especially high population number to a group, for example, a numberabove the maximum population normally allowed.

In such an embodiment, it is advantageous for the base to have means toencourage use of the special groups by individual substations for thepurpose of, for example, sending a backlog of data, but thereafter todiscourage other substations by immediately amending the level ofallocated substations until the backlog has been cleared.

The system may also respond so as to give priority to a substation thatneeds to transmit an urgent message at any time by allowing thatsubstation to associate itself with a group having a low population ofsubstations so that the probability of a successful transmission isincreased. This can be achieved in a similar manner as when a substationprepares to re-transmit following a failure.

DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example with reference tothe accompanying drawings in which:

FIG. 1 is a schematic diagram of a multiple access communications systemaccording to one embodiment of the invention,

FIG. 2 is a more detailed schematic diagram of the base station and onesubstation of the system of FIG. 1, and

FIG. 3 is a schematic representation of the frame composition of the twochannels in the system of FIG. 1 for the transmission of data from thebase to a plurality of substations and from each substation to the base.

THE BEST MODE OF CARRYING OUT THE INVENTION

The illustrated system comprises a base station A and `n` substations ormobiles Bl to Bn. The base station A has a radio transmitter Tx throughwhich it communicates with all of the substations on a first radiochannel C1, and the substations Bl to Bn all communicate with the basestation A on a second radio channel C2 which is received by a radioreceiver Rx at the base station. A control processor P1 is provided atthe base station and responds to communications received on channel C2from the substations, and transmits corresponding communications onchannel C1 to the substations.

Each substation Bl to Bn, as shown in FIG. 2, comprises a radio receiverrx to receive communications on channel C1 from the base station A, anda radio transmitter tx to transmit communications on channel C2 to thebase station. A control processor P2 responds to data received from thebase station A and serves to determine the time slots used by thesubstations to communicate with the base station on channel C2.

Both radio channels C1 and C2 transmit data arranged within frames, asshown in FIG. 3, and each frame in channel C2 is subdivided into t slotsfor the transmission of data from the substations Bl to Bn. Generally,all the frames of each, and preferably the frames of both channels, areof fixed length or integral multiples thereof, and the frames of bothchannels are synchronized either in phase, as shown in FIG. 3, or out ofphase.

In operation of the system, the substations Bl to Bn using the system atany one time are divided into K groups l to k, and each group isassigned to a particular one or more of the slots l to t in the secondchannel C2. Each substation in a group then makes use of the sameassigned slot or slots to communicate with the base station A.

The group to which each substation is assigned is determined by datawhich is transmitted from the control processor P1 to the controlprocessor P2 and include data corresponding to the number ul to uk ofsubstations currently assigned to each of the groups l to k. Forexample, group g, shown in FIG. 3, contains Ug substations which all usethe slot tv, and these group population data Ug are stored in bothcontrol processor P1 and P2.

If a substation fails to communicate successfully with the base stationA on the first attempt or after a predetermined number of attempts usingone or more of the slots l to t assigned to its group, then thesubstation can improve the probability of a successful re-transmissionin a subsequent attempt by selecting a time slot from a pool of timeslots using a random number generator G, as shown in FIG. 2, or adistribution algorithm in the control processor P2.

The pool of time slots can be drawn from those one or more groups withina particular frame and/or those of a group in more than one frame. Forexample, a pool could be composed of the slots assigned to two groupsover two consecutive frames, say group 5 with one slot and group 9 withtwo slots, giving a pool of six slots.

The number of slots in the pool can be increased with successivecommunication failures by increasing either the number of frames and/orthe number of groups within a frame so that the probability of asuccessful communication is progressively increased with the number ofattempts. This increase in probability of success is obtained at theexpense of the first time success rate of those substations whose groupsand frames are actually used in the pool.

The selection of particular groups which are used to form a pool wouldnormally start with any special groups reserved by the control processorP1 of the base section A, a special group being reserved by assigning anespecially high population number U to the group, for example, apopulation number above the maximum population number normally allowedby the control processor P1. After special groups, the pool would drawon those groups with no or few substations assigned to them. The poolcould then be expanded by including the group to which the particularsubstation is currently assigned, and then in a progressive manner thosegroups with higher populations of substations, although it is unlikelythat groups with more than a few substations would ever be used for thispurpose.

In some operating conditions where, for example, too many substationsattempt to use slots assigned to a particular group, the controlprocessor P1 of the base station A can be adapted to set the substationpopulation U for that group artificially high so as to deter use bysubstations from other groups.

In carrying out their operations it will be appreciated that theprocessors P1 and P2 allocate information and/or identification codes tothe slots. Such information and/or identification codes commonlyinclude:

i) means to synchronize the slots of the channels either "in phase",i.e. so that the slots of the channels begin and end simultaneously, or"out of phase", i.e. so that the slots of the channels are offset by apredetermined amount of time,

ii) means to identify the group to which the slot has been assigned,

iii) control information such as a destination address or addresses,

iv) source information if, for example, the origin of data (or whatever)being sent in the slot would otherwise be unclear.

The use of such information/identification means associated with theslots is known to those skilled in the art.

We claim:
 1. A multiple access communications system for use between abase and a plurality of substations and having a first channel fortransmission of data from the base to the substations and a secondchannel for transmission of data from the substation to the base,wherein the system has, in use:i) assigning means at the base forassigning each of the substations to one of a plurality of groups, ii)allocating means at the base for allocating each of a plurality of slotsin the second channel to a particular group, iii) reporting means at thebase for reporting to the substations via the first channel the numberof substations in each group, and iv) analyzing means at each substationfor analyzing the numbers of substations in each of the groups and formaking a judgment based on these numbers as to which group to assigneach respective substation to in order for that substation tocommunicate next with the base in the slot or slots allocated to thatgroup.
 2. A system as claimed in claim 1, wherein said analyzing meansassigns each substation (Bl to Bn) to a particular group (l to k) basedon said judgment of numbers of substations in each group in order thateach substation can re-transmit to the base (A) following a failure tocommunicate.
 3. A system as claimed in claim 1, wherein said analyzingmeans assigns each substation (Bl to Bn) to a particular group (l to k)based on said judgment of numbers of substations in each group in orderthat each substation can communicate an urgent message to the base (A).4. A system as claimed in claim 1, wherein said analyzing means assignssaid substation to that particular group (l to k) having the lowestnumber (ul to uk) of substations in it.
 5. A system as claimed in claim4, wherein each substation (Bl to Bn) has random selection means (G)that operates at random to assign said substation with one of two ormore groups (l to k) that have the same lowest number (ul to uk) ofsubstations in them.
 6. A system as claimed in claim 1, wherein eachrandom substation (Bl to Bn) has random selection means (G) thatoperates at random to introduce a time delay in allowing a substation tocommunicate with the base (A) following a first or subsequent failure ofattempt to communicate with the base station.
 7. A system as claimed inclaim 1, wherein said assigning means (P1) is adapted to assign anominal number (ul to uk) of substations (Bl to Bn) to any group whichdiffers from the actual number of substations in group, thereby toinfluence said assignment of further substations to that group.
 8. Asystem as claimed in claim 1, wherein said assigning means (P1) isadapted to assign a nominal number (ul to uk) of substations (Bl to Bn)to any group (l to k) which number is above a maximum possible number ofsubstations in a group so as to indicate that these are special groupsfor use by the substations under predetermined conditions.
 9. A systemas claimed in claim 1, wherein said judgment based on numbers ofsubstations (Bl to Bn) in each of the groups (l to k) involves theanalyzing means selecting one or more groups so as to produce a pool ofslots (t) associated with these selected groups, random selection meansat each substation then selecting one of the slots (t) in said pool atrandom and assigning the respective substation to this selected slot inorder to communicate next with the base.
 10. A system as claimed inclaim 9, wherein the analyzing means selects groups so as to produce aprogressively larger pool of slots with successive failures tocommunicate by a substation, thereby to increase the probability of asuccessful communication at each successive attempt to communicatefollowing a failure.
 11. A system as claimed in claim 9, wherein saidone or more groups selected are those with lower numbers of substationsin them.
 12. A system as claimed in claim 9, wherein said one or moregroups selected include a special group reserved by the assigning meansassigning a high nominal number of substations to the group.
 13. Asystem as claimed in claim 1, in which said assigning means (P1) isadapted to assign any substation (Bl to Bn) joining the system to saidone group (l to k) according to the number of substations in each group.14. A method of operating a multiple access communications system foruse between a base and a plurality of substations and having a firstchannel for transmission of data from the base to the substations and asecond channel for transmission of data from the substation to the base,comprising the following steps:i) assigning each of the substations toone of a plurality of groups (l to k), ii) the base allocating each of aplurality of slots (l to t) in the second channel to a particular group(l to k), iii) the base reporting to the substations (Bl to Bn) via thefirst channel (C1) the number of substations in each group (l to k), andiv) each substation (Bl to Bn) analyzing the number (Ul to Uk) ofsubstations in each of the groups l to k) and making a judgment based onthese numbers as to which group (l to k) to assign itself to in ordernext communicate with the base (A) in the slot or slots (t) assigned tothat group.
 15. A method as claimed in claim 14, wherein each substation(Bl to Bn) assigns itself to a particular group (l to k) based on saidjudgment of numbers of substations in each group in order to re-transmitto the base (A) following a failure to communicate.
 16. A method asclaimed in claim 14, wherein each substation (Bl to Bn) assigns itselfto that particular group (l to k) having the lowest number (ul to uk) ofsubstations in it.
 17. A method as claimed in claim 14, wherein saidjudgment based on numbers of substations (Bl to Bn) in each of thegroups (l to k) involves selecting one or more groups so as to produce apool of slots (t) associated with these selected groups, and thenselecting one of the slots (6) in said pool, the respective substationusing this selected slot in order to next communicate with the base. 18.A method as claimed in claim 17, wherein groups are selected so as toproduce a progressively larger pool of slots with successive failures tocommunicate by a substation, thereby to increase the probability of asuccessful communication at each successive attempt to communicatefollowing a failure.
 19. A method as claimed in claim 17, wherein saidone or more groups selected are those with lower numbers of substationsin them.
 20. A method as claimed in claim 14, wherein said one or moregroups selected include a special group reserved by the base assigning ahigh nominal number of substations to the group.
 21. A method as claimedin claim 14, wherein a nominal number (ul to uk) of substations (Bl toBn) is assigned to any group which differs from the actual number ofsubstations in the group, thereby to influence assignment of furthersubstations to that group.
 22. A method as claimed in claim 14, whereinthe base assigns each of the substations to one of a plurality ofgroups.
 23. A method as claimed in claim 14, wherein the substationseach assign themselves to one of a plurality of groups on joining thesystem.
 24. A multiple access communications system for use between abase and a plurality of substations and having a first channel fortransmission of data from the base to the substations and a secondchannel for transmission of data from the substation to the base,characterized in that the system has, in use:i) assigning means at thesubstations for assigning each of the substations to one of a pluralityof groups, ii) allocating means at the base for allocating each of aplurality of slots in the second channel to a particular group, and iii)reporting means at the base for reporting to the substations via thefirst channel the number of substations in each group, the assigningmeans analyzing the numbers of substations in each of the groups andmaking a judgment based on these numbers as to which group to assigneach substation to on joining the system.